Many displacement measuring devices are known in the prior art. For example, in Spencer U.S. Pat. No. 2,861,345, a displacement measuring device is disclosed that utilized two diffraction gradings interposed between a light source and a light detector for detecting changes in a Moire fringe pattern as the diffraction gradings move relative to one another, in correspondence to the movement of a workpiece. Similarly, Cail U.S. Pat. No. 2,857,802 teaches the use of two diffraction gradings and a pair of photocell detectors for detecting movement of a workpiece.
In Sanford et al. U.S. Pat. No. 3,713,139, the displacement of a movable member relative to a fixed member is measured via a Moire fringe device for producing cyclic wave patterns upon movement of the movable member. A plurality of light sensors are included for detecting changes in the light wave pattern from the Moire fringe device, for producing signals that are processed by a logic network for determining or measuring the extent of movement and direction of movement of the mechanical member.
Another system for determining the extent of movement of a mechanical member, and/or the direction of movement, are disclosed in Erickson U.S. Pat. No. 3,768,911 which teaches the use of a centrally located double grading between two associated single defraction gradings, for providing a pair of Moire fringe-generating grading pairs in optical series, whereby signals from the pairs are added together for eliminating the need for a collimated light source, and for reducing the optics required in the system, in providing measurement of the motion of a mechanical member. Also, in Post U.S. Pat. No. 3,796,498, a single Moire fringe device including a light source on one side of a defraction grading, and a light detector on another side of the grading, for producing signals upon movement of a scale grading attached to a movable table, whereby an electronic control circuit converts the signals to pulses which are counted for determining the position of the table and providing positioning signals to a table motion control system for positioning the table in a predetermined manner. Other references teaching the use of some form of diffraction grading system or device for linearly determining the movement of a mechanical member are found in MacGovern U.S. Pat. No. 3,812,352; Burns et al. U.S. Pat. No. 3,833,303; Hock U.S. Pat. No. 3,891,321; Kaul et al. U.S. Pat. No. 4,176,276; and Grendelmeier U.S. Pat. No. 3,599,004.
Many devices and systems are also found in the prior art which detect the movement of a mechanical member for determining the rate of flow of a fluid material. For example, in Stenzel U.S. Pat. No. 3,150,360, a plunger partially housed in a cylinder through which gases flow, changes position relative to the rate of flow of the gas. A portion of the plunger is provided outside of the area of gas flow for movement between a light source and a photodetector, thereby permitting electro-optic detection of a range of movement of the plunger for determining high and low flow rates of the gas. A similar device is disclosed in Wiegmann U.S. Pat. No. 3,564,910 for detecting the rate of flow of a predetermined quantity of liquid from a cylinder, for determining the fuel comsumption of internal combustion engines.
In Adelman U.S. Pat. No. 3,156,115, a chopper disc is rotated by a fluid pump for either periodically interrupting light to a light sensor, or periodically changing the reluctance before a magnetic pick-up head, for producing electrical signals that are processed by a controller for providing an indication of the rate of fluid flow through the pump. In Kissel U.S. Pat. No. 3,814,935 a flow meter is provided by including a turbine wheel within the fluid flow path, whereby the turbine is spun by the fluid at a rotational speed proportional to the rate of flow of the fluid, while the turbine is used to chop light from a light source via reflection of the light from the turbine blades to a photodetector, for providing electrical signals analogous to the rate of flow of the fluid. Another flow meter disclosed in Heath et al. U.S. Pat. 4,362,052 includes a piston and cylinder arrangement, wherein fluid flow causes movement of piston, which in turn moves a rod connected to a linear capacitor, for changing the capacitance thereof, thereby providing a measurement via the changing capacitance of fluid flow, where changes in the capacitance are proportional to the movement of the piston.
In Head et al. PCT Publication No. W085/01800, a system for ultrasonically detecting the position of a piston within a cylinder is disclosed. An ultrasonic signal is transmitted from a transducer mounted on the wall of the cylinder to a target zone on the piston, whereby the length of time for the transducer to receive back a reflected signal is measured via a microprocessor for providing digital signals indicative of the position of the piston within the cylinder from some datum point.
In known fluid dispensing systems, such as gasoline pumping systems, it is required that quantity of fluid dispensed to a customer be accurately measured, in order to fairly price the fluid delivered. In a typical gasoline distribution system, such as found in service stations, gasoline is pumped under pressure via a motorized pump to a positive placement meter. As the gasoline is forced through the meter, this causes a pair of pistons within the meter to move in a reciprocating manner, wherein the measurement of the extent of movement of the pistons is indicative of the volume of fluid passed through the meter prior to delivery to a nozzel for distribution to a customer. Many such systems convert the linear motion of the pistons to a rotary motion via cams or cam shafts for rotating a mechanical member, such as a chopper wheel or disc, for example, to "chop" light between a light source and a photodetector, for providing electrical signals corresponding to the movement of the pistons. Such systems are subject to error as mechanical wear in the meter components causes changes in the stroke length of the pistons, which changes are not compensated for via the linear to rotary conversion system.